I tried rebuilding media-gfx/imagemagick, gnome-base/librsvg-2.36.4, and app-text/qpdfview, but building fluxbox always failed on the same icon during the menu generation step.

I've come up with two work-arounds.

(1) Move /usr/share/qpdfview/qpdfview.svg to a temporary location while building fluxbox, and move it back afterwards.

(2) Update to the testing version of dev-libs/glib (I keyworded 2.36.4-r1), along with the required dependency testing versions of dev-libs/gobject-introspection, media-libs/harfbuzz, x11-libs/pango, and x11-libs/pangox-compat (all required by using the testing version of glib).

In the end, I settled on option (1) because I became worried about using the testing version of just a handful of core libraries mixed in with mostly stable versions.